Tetrahydrofuran clusters

In the discussion of the relative acidity of carboxylic acids in Chapter 1, the thermodynamic acidity, expressed as the acid dissociation constant, was taken as the measure of acidity. It is straightforward to determine dissociation constants of such adds in aqueous solution by measurement of the titration curve with a pH-sensitive electrode (pH meter). Determination of the acidity of carbon acids is more difficult. Because most are very weak acids, very strong bases are required to cause deprotonation. Water and alcohols are far more acidic than most hydrocarbons and are unsuitable solvents for generation of hydrocarbon anions. Any strong base will deprotonate the solvent rather than the hydrocarbon. For synthetic purposes, aprotic solvents such as ether, tetrahydrofuran (THF), and dimethoxyethane (DME) are used, but for equilibrium measurements solvents that promote dissociation of ion pairs and ion clusters are preferred. Weakly acidic solvents such as DMSO and cyclohexylamine are used in the preparation of strongly basic carbanions. The high polarity and cation-solvating ability of DMSO facilitate dissociation... 

Neutral metal carbonyl clusters exemplified by Ir4(CO)i2, IrelCOlie, and Rh6(CO)i6 are adsorbed intact from solution (e.g., n-pentane) onto more-or-less neutral supports such as y-Al203 or Ti02. The clusters on these supports can often be extracted intact into solutions such as tetrahydrofuran. 

General Methods. Methanol used in kinetic runs was distilled from sodium methoxide or calcium hydride in a nitrogen atmosphere before use. Freshly distilled cyclohexanol was added to the methanol in the ratio 6.0 ml cyclohexanol/200 ml MeOH and was used as an internal standard for gas chromatographic (GC) analysis. Benzaldehyde was distilled under vacuum and stored under nitrogen at 5°. Other aldehydes (purchased from Aldrich) were also distilled before use. The corresponding alcohols (purchased from Aldrich) were distilled and used to prepare GC standards. All metal carbonyl cluster complexes were purchased from Strem Chemical Company and used as received. Tetrahydrofuran (THF) was distilled from sodium benzophenone under nitrogen before use. 

In general the deprotonation of a polyborane B H +m (to = 4,6) leads to the anions [BnHn+m-i] or [B H +m 2]2 by removal of one or two protons from a BHB 3c2e bridge with formation of a B-B single bond. Cluster expansion with a BH3 unit, usually offered as diborane in diethyl ether or tetrahydrofuran, produces borane anions [B +1H +m+2] or [B +1H +m+1]2 and these in turn on protonation give the polyboranes Bn+iHn+m+-. These may be stable species. However, in most cases they loose H2 which results in a cluster expansion by one BH unit. Several examples of this method are described in the following sections. 

On a preparative scale, 4 and 5 can be made from the appropriate neutral parent cluster by reduction with two equivalents of sodium metal in tetrahydrofuran or, more conveniently, by the addition of excess sodium carbonate or potassium hydroxide to 1 or 2 in methanol.15... 

Pyrolysis of Et4N[ReH2(CO)4] at 250°C in n-tetradecane yields a mixture of polynuclear products from which two carbidocarbonyl clusters of rhenium have been isolated. Extraction of the pyrolysis product mixture with tetrahydrofuran results in a residue of (E N ReyCfCO iJ, 34 (76), and a solution from which may be isolated (Et4N)2[Re8C(CO)24], 35, (77), the structures of which are shown in Figs. 42 and 43. Both are based on an... 

There are no structurally characterized cluster complexes containing /t2-vinylidene ligands, although several of the minor products isolated from reactions between Ru3(CO),2 and NaBH4 in refluxing tetrahydrofuran are supposed to have this feature (117) examples are complexes 73-75. 

Treatment ofRu3(CO)11(Ph2PC=CPr ) in tetrahydrofuran with water or ethanol affords up to 30% of the red tetranuclear clusters (90 R = H or Et, respectively) (135). In these, the cleavage of the P-C bond is accompanied by addition of OH or OEt to the cluster, with transfer of hydrogen to the / -carbon. Unusually, the hinge metal atoms are separated by 3.455 A, too far for a normal metal-metal interaction this has the result of bringing the HyO and/i4-C atoms to within 2.65 A of each other, suggesting the possibility of facile H transfer between the two atoms. 

Fe]3 ) in protic solvents. The reduced species of the clusters are generated by controlled potential electrolysis (c.p.e.) at a Hg electrode using a potential that will form [4-Fe]3, 4-Fc 4, or Mo-Fe]5. Although N2 reduction was observed in such systems, the current efficiency was very low, with a maximum of 1.6% for the [4-Fe]3 cluster in methanol/tetrahydrofuran (MeOH/thf). The total yield of NH3 based on the cluster was 195% for 4 days. It is surmised that most of the electrons transferred from the electrode are consumed in H2 evolution from the solvent. 

The first product of this reaction, which should be compared with Eq. (18) in Section II,A,3, is a derivative of the well-known —CCos(CO) cluster, with a tetrahedral CCo3 grouping the product in Eq. (99) is similar. The second product in Eq. (100) is clearly derived from attachment of tetrahydrofuran to a trimethylsilyl group, followed by rearrangement. 

The compound Na2 [Rh12(CO)30] can be prepared by reaction of Rh2(CO)4-Cl2 with sodium acetate in methanol under an atmosphere of carbon monoxide.1 It contains one of the fust polynuclear anions to be formed when the rhodium carbonyls or carbonyl halides are reduced by the action of alkaline reagents in alcohols or by alkali metals in tetrahydrofuran (THF). It provides a unique example of a double octahedral cluster carbonyl anion in which the noble gas rule is not obeyed,1 2 and it is a starting material for the preparation of other polynuclear rhodium carbonyl anions.1 3"5 The synthesis reported here is a modification of the original method. The starting material is Rh4(CO)i2, now easily prepared at atmospheric pressure.6"8 The reaction is fast, and the overall procedure requires about 6-7 hours with 80-85% yields. 

The incorporation of a main group atom derived from the reducing agent itself has also been noted in a few cases. For example, reduction of Rh4(CO),2 with SCN in isopropanol has been found to give the sulfido cluster [RhioS(CO)22] in high yield 203), while [Ru6N(CO),6] was produced from the reduction of Ru3(CO)i2 with Nj in refluxing tetrahydrofuran 24). 

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